1. Field of the Invention
The present invention relates to an induction cooking heater of the type comprising at least one inductor and magnetic field concentrator located beneath the inductor.
2. Description of the Related Art
Induction cooking heaters use half-bridge converters for supplying the load composed of the system induction coil+cooking vessel in series with two parallel resonant capacitors. As indicated in the attached FIG. 1, the power transistors commutate the rectified line voltage and output it to a RLC load circuit, which is the equivalent electrical model of the series connection of coil-pot and resonance capacitor.
The magnetic coupling of the coil-pot assembly can be modelled as a transformer with two secondary windings: one corresponds to the pot bottom and the second one corresponds to the magnetic field concentrator, usually in the form of ferrite bars or the like placed beneath the induction coil. The main function of these ferrite bars is to focus (i.e. concentrate) the magnetic field lines generated by the inductor and prevent them to pass through the aluminium plate support.
In the attached FIG. 2 it is shown a cross section of a usual induction heating cooktop, where the magnetic field vectors are schematically shown.
Based on the Ampere Law:
            ∑      area        ⁢    I    =      ∮          H      ⋆              ⅆ        l            the equivalent electrical model of the coil-pot assembly is shown in FIGS. 3 and 4.
As the ferrite bars concatenate the electro-magnetic fields generated by coil current of the induction heating half bridge converter, they start self-heating due to (mainly) the hysteresis energy loss.
The hysteresis power loss depends on frequency, the ferrite volumes and the maximum magnetic field B, as described in the below empirical Steinmetz equation:Physt=Kh*f*Bmaxα
The magnetic relative permeability changes non-linearly with the temperature at ferrite core.
FIG. 5 shows relative permeability vs. temperature curve of a standard commercial ferrite used for coil inductors. As shown in the figure, the relative permeability increases with temperature and reaches maximum temperature at around 225° C.
At that point, if power is not reduced and ferrite bars keep on self-heating, they may reach the Curie-point temperature at which any ferromagnetic material becomes paramagnetic, and so it becomes “transparent” to magnetic field (i.e. the relative permeability “collapses”). Then, being the ferrites “transparent” for magnetic fields, this magnetic field will pass through the aluminium plate support, which is a highly electrical conductive non-magnetic material. Induced current starts flowing through the aluminium plate.
This sharp transition from ferromagnetic to paramagnetic characteristic changes the equivalent electrical model of the coil load as seen from the power converter side: the electrical complex impedance at coil terminals is reduced considerably. Somehow, it would be as the magnetic inductance is short-circuited. Then the reactive and resistive part of the complex impedance of the load (inductor coil−pot assembly) will be equal to the dispersion inductance and coil winding resistance.
This new equivalent impedance load seen at coil terminals is connected in series with the resonant capacitors: the total impedance supplied with commutated rectified line voltage (whose fundamental voltage component is equal to 93.2 Vrms when line voltage is 230 Vrms) is too small and that makes the output current rises sharply.
These working conditions should be avoided before reaching them by reducing the output power. In the case of standard induction heating cookers, if such condition is not early detected, then the power transistors of half bridge converter might fail due to the high current during conduction time.
There are several ways of detecting it, for instance today a standard method measures the ratio of maximum current and rms (root mean square) current and compare it with a predefined threshold value. As can be seen in the figures, normally the phenomenon of inductor ferrite saturation starts at the peak of output commutated voltage (that corresponds in time with line voltage peak). The peak loss power that is dissipated by ferrite cores at these points is enough for heating the ferrite up to Curie-point (becoming it paramagnetic).
Once the output commutated voltage amplitude is lower than a certain value, the ferrite core cools down and its relative permeability “recovers” the value previous to saturation. This phenomenon can be detected easily by measuring the coil current. This method is also disclosed by U.S. Pat. No. 5,665,263 where controls are provided for detecting the surge of current flowing into the inductor when the ferrite bars have reached magnetic saturation.
Also EP-A-209215 discloses a temperature regulating apparatus that includes sensing coil for monitoring the change in permeability that occurs when a ferromagnetic element has reached its Curie temperature, the power cut-off or reduction being carried out only after this detection. The above method needs to supply constant ac voltage or constant power to the sensing coils which might increase the cost of the hardware components and it cannot be applied in other areas as induction heating cooktops where the load to be heated cannot be wound around by sensing coils.
The above known methods needs that ferrite saturation happens (and so related over current). Moreover according to these known methods it is not possible to assess which are the optimum working conditions where ferrite bars are heated up to the temperature closer to the Curie-point where relative permeability has still an acceptable value controlling the output power so as to avoiding the saturation of the ferrites.